The present invention relates to a plant for transmitting electric power comprising a semiconductor device of turn-off type connected in series with a conductor and a member connected in anti-parallel therewith able to assume a state in which it blocks a conduction of current therethrough when a voltage is applied over the power semiconductor device in the conducting direction thereof and a state in which it conducts a current therethrough when a voltage is applied over the power semiconductor device in the opposite direction.
xe2x80x9cA plant for transmitting electric powerxe2x80x9d is to be given a very broad sense and is intended to comprise in principle all equipment and the like in which electric power is fed in a conductor.
However, the invention is particularly well applicable to the case of transmitting high electric powers through high voltage, in which high voltage is typically a voltage between 2 and 400 kV, and then primarily in connection with stations provided with converters for converting the voltage with respect to level and/or type (direct voltage, alternating voltage), and for that reason the particular case of a plant having a VSC-converter for converting direct voltage into alternating voltage or direct voltage and conversely will hereinafter be described for illuminating, but accordingly not in any way restricting, the invention and the problem forming the basis therefor.
A plant of the type defined in the introduction is schematically illustrated in appended FIGS. 1 and 2, said plant having a VSC (Voltage Source Converter)-converter, which here is constituted by a so called 6-pulse bridge having three phase legs 1-3, each consisting of two current valves 4-9 connected in series, which in their turn are formed by a first power semiconductor device 10 of turn-off type and a rectifying member 11 in the form of a so called free-wheeling diode connected in anti-parallel therewith. Each valve comprises in practise a comparatively high number of power semiconductor devices of turn-off type connected in series and free-wheeling diodes for enabling the valve to take the high voltages it has to take in the blocking state. The midpoints between the two valves of the respective phase leg are at a phase output 12 connected through a phase reactor 13 to a phase each of a three phase alternating voltage network 14. The phase legs are at the ends thereof connected to a pole conductor 15, 16 each of a direct voltage network 17, the voltage of which is defined through two capacitors 18, 19 connected in series and having a grounded midpoint 20.
The power semiconductor devices 10 of turn-off type are controlled in a way known per se through a control arrangement 21 according to a determined pulse width modulation pattern (PWM) for using the direct voltage over the DC-capacitors 18, 19 to generate a voltage on the phase output 12, the fundamental tone component of which constitutes an alternating voltage having a desired amplitude, frequency and phase position. The switching frequency of the power semiconductor devices 10 is then usually 1-3 kHz, but the frequency of the alternating voltage will be 50 or 60 Hz. The plant has also a breaker 33 adapted to close and open, respectively, the connection between the alternating voltage network and the converter, and which is closed when a voltage is supplied to the converter after interruption of the operation.
The principle of a VSC-converter already known and just described implies that the direct voltage is higher than the peak value of the alternating voltage, i.e. the diodes 11 connected in anti-parallel are reversed biased when the converter is blocked. When a ground fault occurs on the direct voltage side, which is schematically indicated at 22, this means that the diodes 11 will be excerted to a short circuit current having a peak value being approximately 2,5 times higher than a symmetrical short circuit current until the breaker 33 therebehind has opened, which takes place after 2-3 periods. The blocking of the 6-pulse bridge does not prevent the fault current as schematically indicated at 23. This means in the practise that the users require that the diodes have to be able to take said current therethrough during a considerably longer time, maybe 12-20 periods, for having a sufficient safety margin, which means that unreasonable demands are put on the diodes, which have to be heavily over-dimensioned for satisfying this and they will by that be very costly.
Another problem adhered to a converter of this type concerns the putting of the converter under voltage after an interruption of the operation. When the alternating voltage breaker in such a case is closed the DC-capacitors 18, 19 are charged through the diodes 11 in the valves. Energy stored in the phase reactor 13 when the capacitors are charged will raise the direct voltage further, in the worst case to a duplicating of the direct voltage. This problem may be solved by using a so called switching-in-resistance, but that solution is costly and it is desired to improve the result thereof.
It is shown in FIG. 2 how a so called quick disconnector 24 is arranged between the direct voltage network 17 and the converter for the respective pole conductor. The direct voltage network has in this case a so called meshed fashion with a system of a plurality of stations with converters connected in parallel to the direct voltage network. The quick disconnector 24 is formed by a power semiconductor device 25 of turn-off type and a diode 26 connected in anti-parallel therewith and has the object to isolate the converter at a ground fault on the direct voltage network 17, so that the converter will not feed a large current directly into such a ground fault. However, the presence of the diode 26 means that the quick disconnector 24 will be conducting in one direction. This results in its turn in the fact that if a ground fault 27 schematically indicated occurs within the converter, all the converters connected to the system will feed current into the fault, such as indicated through the dashed lines 28-30. If all the stations of the system, as is the case in FIG. 2, are provided with quick disconnectors of their own they will open as a consequence of the overcurrent and this means that all transmission on the direct voltage network will be interrupted as a consequence of an internal fault in one of the stations, which of course is unacceptable.
The object of the present invention is to provide a plant for transmitting electric power of the type defined in the introduction, which has a function improved in certain respects with respect to such plants already known and enables a reduction of the inconveniences discussed above.
This object is according to the invention obtained by providing such a plant, in which said member is formed by a controllable second power semiconductor device having the conducting direction opposite to that of said first power semiconductor device of turn-off type.
Through this totally new approach to use a controllable power semiconductor device as a member connected in anti-parallel with a power semiconductor device of turn-off type a number of advantages in different operation situations are obtained. By arranging the controllable second power semiconductor device the flexibility is considerably increased, since it will be possible to design the unit formed by the two power semiconductor devices to either block when a voltage is applied over the power semiconductor device of turn-off type in the blocking direction or conduct by controlling the second power semiconductor device to conduct. Thus, an optimum adaption to the operation situations prevailing may be obtained.
According to a preferred embodiment of the invention, which is applicable to a plant comprising a VSC-converter for converting direct voltage into alternating voltage or direct voltage and conversely and which has at least one phase leg with two current valves connected in series, and in which a point of the phase leg between said valves is intended to be connected to a phase of a direct or alternating voltage network and the opposite ends of the phase leg are intended to be connected to a pole conductor each of a direct voltage network, the valves are formed by at least one said first power semiconductor device and a second power semiconductor device connected in anti-parallel therewith. This means that in the case discussed above of a ground fault on the direct voltage side the short circuit current may through the converter to the ground fault be eliminated very rapidly by controlling the second power semiconductor device in a corresponding way. This means that in the case that this power semiconductor device is a controllable power semiconductor device being not of turn-off type, such as a thyristor, the current could be eliminated after a maximum of one period, which may be compared with the considerably longer time it will take before the so called back-up-breaker located therebehind between the converter and the alternating voltage network may break or interrupt. Would this second power semiconductor device be of turn-off type the fault current could be eliminated even more rapid.
According to another preferred embodiment of the invention the second power semiconductor device of the valve is controllable but not of turn-off type. An advantage of using such a power semiconductor device, which preferably is a thyristor, in the valve instead of a power semiconductor device of turn-off type is that it is not required that the energy stored in inductances, the alternating voltage reactor or transformers has to be dealt with by for example capacitors between the phases.
According to another preferred embodiment of the invention the plant comprises a switch having the first and the second power semiconductor device, and the first power semiconductor device of turn-off type is adapted to conduct current during normal function of the plant and to be controlled to be turned off for fulfilling a disconnector function when this is desired. By arranging a first power semiconductor device of turn-off type and a controllable second power semiconductor device connected in antiparallel therewith in this way for fulfilling said disconnector function the problems discussed above at ground faults within a converter can be solved in a satisfying way. Thus, by controlling the second power semiconductor device to the blocking state simultaneously as the first power semiconductor device is turned off in the disconnector arranged at the converter in question it may be prevented that current is fed into the fault in question from the direct voltage network and from possibly other converters. This means in its turn that no over-current will occur in the other converters in the case of a multiple station system, so that the transmission on the rest of the network will continue as if the fault has never occurred. This also constitutes other preferred embodiments of the invention.
According to another preferred embodiment of the invention the second power semiconductor device is of turn-off type, which is particularly advantageous in the case when the two power semiconductor devices belong to a switch having a disconnector function, since the disconnecting may then take place very rapidly and the two components of turn-off type may then be controlled by the same order, which simplifies the control electronic.
According to another preferred embodiment of the invention, which relates to a plant according to above having a VSC-converter, the plant comprises an arrangement adapted to control the turning on of the second power semiconductor device when a voltage is applied to the converter and by that the voltage increase on the direct voltage side. It will by this be possible to manage without any switching-in-resistance mentioned above, and the voltage increase may nevertheless be kept within acceptable limits by controlling the turning on, i.e. the control angle of the power semiconductor device, preferably a thyristor.
According to another preferred embodiment of the invention the controllable second power semiconductor device is adapted to be in the conducting state at normal operation of the plant. This means that this power semiconductor device at normal operation of the plant will in principle function as a free-wheeling diode, but when desired it may be controlled into the blocking state, i.e. so that it does not conduct in the forward biased direction for avoiding the disadvantages of a rectifying diode in such fault states.
According to another preferred embodiment of the invention the plant comprises an arrangement adapted to control the first and second power semiconductor devices through a light conductor in common by coded orders. A degree of complicity of the control electronic of the plant being essentially unchanged will by this be obtained in spite of the fact that in such a plant the second power semiconductor device is controllable contrary to the case of such plants already known.
According to a preferred embodiment of the invention the controllable second power semiconductor device is a thyristor, in which this may be only controllable and not of turn-off type, or a GTO of turn-off type. The second power semiconductor device could also be constituted by an IGBT.
The first power semiconductor device is preferably an IGBT.
Further advantages as well as advantageous features of the invention will appear from the following description and the other dependent claims.